The present invention relates generally to charging circuits for rechargeable batteries, and more particularly to a safety circuit for rapidly clamping surge current for battery protection while simultaneously allowing for an unimpeded startup of a battery charging circuit.
Many portable electronic devices use a rechargeable battery as a power source. Example portable electronic devices include computers, cellular telephones, pagers, radios, and the like. While there are many types of rechargeable batteries used today, including nickel cadmium and nickel metal hydride, lithium ion batteries have become a popular choice. Lithium ion batteries are typically smaller and lighter than other rechargeable battery types with increased charge capacity.
The charging of lithium ion batteries is conducted in a different manner than the charging of nickel type rechargeable batteries. Generally, nickel-type rechargeable batteries are charged by applying a constant current from a battery charger to the battery until the cell reaches a predetermined voltage or temperature. A lithium ion cell, however, uses a different charging process. First, the lithium ion cell is supplied with a current from a battery charger until the cell""s voltage rises above a threshold. Next, the battery charger is held at the threshold until the current of the cell decreases to a predetermined level. Damage to at least the battery or battery charger may occur when a battery charger applies an improper voltage to a rechargeable cell.
The present invention is directed towards charging circuits. According to one aspect of the invention, a safety circuit for protecting a cell against an over-voltage condition is provided. The circuit comprises a first, second, third, and fourth transistor. The first transistor has a control terminal that is coupled to a first node, a first noncontrol terminal that is coupled to a terminal that is arranged for coupling to the cell, and a second noncontrol terminal that is coupled to a second node. The second transistor has a control terminal that is coupled to a third node, a first noncontrol terminal that is coupled to the second node, and a second noncontrol terminal that is coupled to a low power supply. The third transistor has a control terminal that is coupled to a fourth node, a first noncontrol terminal that is coupled to the second node, a second noncontrol terminal that is coupled to the third node. The fourth transistor has a control terminal that is coupled to a fifth node, a first noncontrol terminal that is coupled to the terminal that is arranged for coupling to a cell, and a second noncontrol terminal that is coupled to the third node.